Drupal site not behaving - process of elimination

I have two versions of a Drupal site running on my local dev environment. Site X boots perfectly; site开发者_运维问答 Y doesn't - rather than running index.php, install.php is called.

So far, I've:

• Stepped through site Y with a PHP debugger; index.php does not seem to be called at all.
• Pointed site Y at the database for site X to see whether there were database corruption issues; seemingly, there weren't.
• Swapped out the .htaccess file in site Y for the .htaccess file in site X. No joy.

What should I try next?

Does settings.php exist in site Y and does it have the correct database information?

Also, are these two seperate drupal installs, or one drupal install setup multisite?

the above provides a walkthrough, so question answered.